Using the recently-developed large and unique Kaiser-Permanente Twin Registry, we propose to carry out a three-phase study of atherosclerotic disease and risk factors in adult like-sex monozygotic and dizygotic twins, age 18 and over. The over-all purpose is: 1. to assess the degree of genetic determination of atherosclerotic disease (particularly coronary heart disease (CHD) and risk factors, and 2. to assess the role of environmental factors in CHD, with emphasis on cigarette smoking and alcohol consumption, controlling for genetic effects. The first phase will be an analysis of responses to an extensive questionnaire on habits, environmental exposures and health status and disease history of self and family, that is being sent to about 3,500 pairs, and to 600 survivors of death-discordant pairs (death certificate of the deceased co-twin will be obtained). The second phase will be a review and analysis of medical records of the 1,400 pairs subscribing to the Kaiser Foundation Health Plan in Northern California. Questionnaire data will be used to relate environmental exposures to disease status determined in the medical records. The third phase will be examination of 432 pairs of adult female twins living in the San Francisco Bay Area, including a clinical and laboratory test panel, blood testing for genetic markers to confirm zygosity, complete determination of plasma lipids and lipoproteins, history and physical examination with confirmation of cardiovascular disease by an internist and treadmill electrocardiogram for subjects with possible or probable angina pectoris. Long-term follow-up by questionnaire and record review is also planned. This would be the first large-scale study of atherosclerotic disease and risk factors in twins which would include non-whites and which would involve an examination of women.